The present invention relates to hydrogen production, particularly to hydrogen production by high temperature steam electrolysis, and more particularly to natural gas-assisted high temperature steam electrolyzers that will lower the electricity consumption to at least an estimated 35 percent of conventional steam electrolyzers.
Hydrogen is a reactant in many industrial processes and is envisaged to become even more important in the future as a chemical reactant, as well as a premium fuel. Presently, most of the total hydrogen demand is met by hydrogen production from fossil fuels; i.e., by steam reforming of natural gas and by coal gasification. Hydrogen produced from water electrolysis is much simpler and has no adverse localized environmental consequences. However, up to the present time, water electrolysis has no significant commercial application because the process requires the use of large amounts of electricity, which results in a high production cost.
From the thermodynamic viewpoint, it is more advantageous to electrolyze water at high temperature (800.degree. C. to 1000.degree. C.) because the energy is supplied in mixed form of electricity and heat. See W. Donitz et al., "High Temperature Electrolysis of Water Vapor-Status of Development and Perspective for Application," Int. J. Hydrogen Energy 10,291 (1985). In addition, the high temperature accelerates the reaction kinetics, reducing the energy loss due to electrode polarization and increasing the overall system efficiency. Typical high temperature electrolyzers, such as the German Hot Elly system, achieved 92 percent electrical efficiency while low temperature electrolyzers can reach at most 85 percent efficiency. See above-referenced W. Donitz et al. Despite the high efficiency, the German system still produces hydrogen at about twice the cost of the steam reformed hydrogen. To promote widespread on-site production of the electrolytic hydrogen, the hydrogen production cost must be lowered. According to the German analysis of the Hot Elly system, about 80 percent of the total hydrogen production cost can be attributed to the cost of electricity needed to run the system. Therefore, to make electrolysis competitive with steam-reformed hydrogen, the electricity consumption of the electrolyzer must be reduced to at least 50 percent for any current system. However, there is no obvious solution to this problem because high electricity consumption is mandated by thermodynamic requirements for the decomposition of water.
The present invention provides a solution to the above-mentioned high electricity consumption in high temperature steam electrolyzers. The invention provides an approach to high temperature steam electrolysis that will lower the electricity consumption to at least 65 percent lower than has been achieved with previous steam electrolyzer systems. The invention involves a natural gas-assisted steam electrolyzer for hydrogen production. The resulting hydrogen production cost is expected to be competitive with the steam-reforming process. Because of its modular characteristics, the system of the present invention provides a solution to distributed hydrogen production for local hydrogen refueling stations, home appliances, and on-board hydrogen generators.